Anxiety disorders, highly prevalent and disabling conditions in children and adolescents, rarely remit and increase the risk of subsequent depression, anxiety, substance abuse, and suicide in adulthood. Available treatments when effective can reduce morbidity, but often yield only modest success. One important strategy would be to guide individual patients towards treatments that have the highest likelihood of treatment success. Although SSRIs are widely used for pediatric anxiety disorders, little is known about how these medications exert their therapeutic effects. This knowledge gap precludes the development of biologically-based, hypothesis-driven breakthroughs to guide patients towards individually-tailored, optimal treatment strategies. This proposal seeks to discover neural markers that can be used to understand how SSRIs exerts their effect and to inform treatment decisions. Published work and pilot studies from the PIs indicate that the neural circuitry (particularly amygdala- ventral prefrontal cortex [vPFC] circuitry) that mediates fear responding is relevant to the pathophysiology of anxiety disorders and may be related to clinical treatment response. In the context of a randomized, placebo-controlled clinical trial of the SSRI sertraline, a widely-used first-line treatment with a variable response rate (pooled estimate of 63%; range: 55-91%) based on controlled clinical trials, this study will perform pre- and post-treatment functional MRI (fMRI) of amygdala and amygdala-vPFC function in children and adolescents (age range: 7-19 years) with anxiety disorders (AD) and healthy comparison (HC) youths. The Aims are: 1) Compare amygdala reactivity and amygdala-ventral prefrontal cortex functional connectivity ('coupling') to signals of threat between children and adolescents with anxiety disorders (AD) and matched healthy control (HC) subjects prior to treatment; 2) Compare the change (pre- treatment vs. post-treatment) in amygdala reactivity and amygdala-vPFC functional connectivity (coupling) between AD youths treated with the SSRI sertraline and those treated with placebo (PBO); 3) Characterize the relationship between pre-treatment amygdala-vPFC functional connectivity (and amygdala reactivity) and subsequent sertraline treatment response in AD youths; and 4) Examine the effects of development on amygdala reactivity and amygdala-vPFC functional connectivity in relation to AD, treatment change, and predictor of treatment response. This approach is innovative as no study has examined the relationship between brain function on treatment response in children and adolescents with anxiety disorders in a placebo- controlled design. The expected outcome of this work will be to help identify the effects of SSRI treatment on brain function and the brain mechanisms that mediate individual differences SSRI treatment response. The broad goal is to help to guide young patients towards treatments with higher likelihood of success, minimize trial-and-error prescribing and speed delivery of effective care to patients. This work will also elucidate anxiety- related brain targets for novel interventions in children and adolescents.